It is not something people say so who knows what it means. If you can't find a number of attestations, I'm going to vote to close the question on the grounds that you're just coining phrases and asking what they mean. It's possible a speaker is using only there as a synonym for just but that is an idiosyncratic use with this particular phrase. Attestations please.
– TᴚoɯɐuoDec 24 '18 at 13:20

... he lifted up her hands and eyes, and said she would try to believe it; but she thought it was only too good to be true.
– ApollyonDec 24 '18 at 13:23

In terms of methodology, your web link would suffice as grounds for a question, but not as an attestation. Use Google Books for attestations, not web pages. We have to be reasonably confident that a native speaker is the author and that the work is intended for an audience of native speakers. Also, always provide at least several attestations to show that the phrase is not a one-off.
– TᴚoɯɐuoDec 24 '18 at 13:25

"Google Books Ngram Viewer shows that its use..." Please cite a few of these attestations of "only too good to be true". Or do you mean something else by "its"?
– TᴚoɯɐuoDec 24 '18 at 17:14

@Tᴚoɯɐuo In spite of some searching, I've yet to find a neat way of citing Ngram results (of the kind I've seen from fumblefingers). Mine run to several lines. Guidance appreciated.
– Ronald SoleDec 25 '18 at 1:02

I don't have a particularly neat way, but if you follow the links at the bottom of the Ngram search page, you will come to a page listing individual texts which may or may not match the pattern. You can visit each of those texts in turn to determine whether it's an actual match, and if so, copy the URL into the answer using the add-link button on the editor.
– TᴚoɯɐuoDec 25 '18 at 1:15

✔ I'm only too happy to help you.
✔ They knew only too well that the bell meant playtime was over.

There is also nothing wrong with other words used in front of too good to be true:

✔ That was justtoo good to be true.
✔ That was simplytoo good to be true.

Both of these sound fine. The first arguably means something that has just crossed the line from believable to unbelievable. Although, in normal usage, it is actually just putting emphasis on something that is too good to be true.

But this sounds wrong to me:

? That was onlytoo good to be true.

I don't see see how only can be used in this construction and make sense. To me, it has the same meaning as:

✘ That was solelytoo good to be true.

While this is technically grammatical, solely too good (as with only too good) is somewhat nonsensical—not least of all because it's not used idiomatically.

Possibly, you could interpret it as:

✔ On its own, that was too good to be true.

Or:

✔ Barring other considerations, that was too good to be true.

However, that would mean that you are reinterpreting the syntactic use of only, and changing it from an adverb (that modifies too) into an implied introductory clause. You'd be replacing what's actually written with something that isn't written at all.